Am I doing something wrong?

My Fiancé can't stay hard most of the time when we're having sex even after I make sure he's turned on and everything. At first it was because he was on pills and now that he's off of them it's happening a lot more. And when he does stay hard he cums so fast. He says it's not my fault but I'm starting to feel like it is. Am I not good enough? Am I doing something wrong? I'm so confused and hurt.